1. Field
The presently disclosed subject matter relates to a piezoelectric actuator including lead titanate zirconate (PZT) and its manufacturing method.
2. Description of the Related Art
Lead titanate zirconate PbZrXTi1-XO3 (PZT), which is an oxide compound including lead (Pb), zirconium (Zr) and titanium (Ti), has a simple cubic-system perovskite crystal structure as illustrated in FIG. 8. In FIG. 8, note that a shaded sphere indicates Pb, a black sphere indicates Zr or Ti, and a white sphere indicates 0. As illustrated in FIG. 9, which is a graph for showing an X-ray diffractive pattern of PZT, PZT generates a polarization when PZT is distorted along its <100> direction or <111> direction, thus exhibiting an excellent piezoelectric characteristic when PZT has an orientation of (100) or (111) (see: FIGS. 5 and 10 of JP2003-81694A). That is, the crystal structure of PZT constitutes a tetragonal-system or a rhombohedral-system. In the tetragonal-system crystal structure of PZT, the largest piezoelectric displacement is obtained along the <100> direction (the a-axis direction) (or the <001> direction (the c-axis direction)), while, in the rhombohedral-system crystal structure of PZT, the largest piezoelectric displacement is obtained along the <111> direction. Also, as to the breakdown voltage characteristic which is an important characteristic for piezoelectric actuators, titanium (Ti)-rich (x<0.5) tetragonal-system PZTs are advantageous over rhombohedral-system PZTs. Therefore, PZT piezoelectric layers using such tetragonal-system PZTs are used for micro electro mechanical systems (MEMS) elements as actuators, MEMS elements as sensors, electricity generating elements, gyro elements and so on.
In FIG. 10, which is a cross-sectional view illustrating a prior art piezoelectric actuator, this piezoelectric actuator is of a laminated capacitor type which includes a monocrystalline silicon substrate 1, a silicon oxide (SiO2) layer 2, a titanium (Ti) adhesive layer 3, a platinum (Pt) lower electrode layer 4, a tetragonal-system PZT piezoelectric layer 5 and a Pt upper electrode layer 6. In this case, the monocrystal line silicon substrate 1 can be replaced by a silicon-on-insulator (SOI) substrate. Also, since the silicon oxide layer 2 has bad adhesion characteristics with the Pt lower electrode layer 4, the Ti adhesive layer 3 is interposed therebetween in order to improve the adhesion characteristics between the silicon oxide layer 2 and the Pt lower electrode layer 4 and relax a stress therebetween.
In FIG. 10, when the direction of the PZT piezoelectric layer 5 as indicated by an arrow is along the <100> direction or the <001> direction, distortion is effectively generated by applying a DC voltage between the Pt lower electrode layer 4 and the Pt upper electrode layer 6.
A method for manufacturing the piezoelectric actuator of FIG. 10 is explained next with reference to FIG. 11.
First, referring to step 1101, a monocrystalline silicon substrate 1 is thermally oxidized to grow a silicon oxide (SiO2) layer 2 thereon. In this case, not that a chemical vapor deposition (CVD) process can be used instead of the thermal oxidization process.
Next, referring to step 1102, a Ti adhesive layer 3 is formed by a sputtering process using Ar gas on the silicon oxide layer 2.
Next, referring to step 1103, a Pt lower electrode layer 4 is formed by a sputtering process using Ar gas on the Ti adhesive layer 3.
Next, referring to step 1104, before an arc discharge reactive ion plating (ADRIP) main process at step 1105 for forming a PZT piezoelectric layer 5, an ADRIP pre-process is carried out in an ADRIP apparatus in a vacuum atmosphere to heat the monocrystalline silicon substrate 1, the silicon oxide layer 2, the Ti adhesive layer 3 and the Pt lower electrode layer 4 to about 500° C. This ADRIP pre-process will be described later.
Next, referring to step 1105, the ADRIP main-process is carried out in the same ADRIP apparatus subsequent to the ADRIP pre-process at step 1104 to form a PZT piezoelectric layer 5. This ADRIP main-process will also be described later.
Finally, referring to step 1106, a Pt upper electrode layer 6 is formed by a sputtering process using Ar gas on the PZT piezoelectric layer 5.
The ADRIP main-process at step 1105 has an advantage in that the deposition speed of PZT is higher than the sputtering process. Also, the ADRIP main-process has an advantage in that the substrate temperature is lower, the manufacturing cost is lower, it is more eco-efficient and more efficient in utilization of materials over the metal organic chemical vapor deposition (MOCVD) process using poisonous organic metal gas.
An ADRIP apparatus used for carrying out the ADRIP pre-process at step 1104 and the APRIP main-process at step 1105 is explained next with reference to FIG. 12 (see: FIG. 1 of JP2001-234331A).
In FIG. 12, provided at a bottom portion of a vacuum chamber 1201 is a Pb evaporation source 1202-1, a Zr evaporation source 1202-2 and a Ti evaporation source 1202-3 for independently evaporating Pb, Zr and Ti, respectively.
The Pb evaporation source 1202-1, the Zr evaporation source 1202-2 and the Ti evaporation source 1202-3 are associated with a Pb evaporation amount sensor 1202-1S, a Zr evaporation amount sensor 1202-2S and a Ti evaporation amount sensor 1202-3S, respectively, for detecting Pb, Zr and Ti evaporation amounts within the vacuum chamber 1201.
Also, provided at an upper portion of the vacuum chamber 1201 is a heater incorporating wafer rotating holder 1203 for mounting a wafer 1203a. 
Further, provided at an upstream side of the vacuum chamber 1201 are a pressure gradient type arc discharge plasma gun 1204 for introducing insert gas such as Ar gas and He gas thereinto and an O2 gas inlet pipe 1205 for introducing O2 gas thereinto as material for the PZT piezoelectric layer 5. The amount of O2 gas introduced into the vacuum chamber 1201 is adjusted by an adjusting valve 1205a. On the other hand, provided at a downstream side of the vacuum chamber 1201 is an exhaust pipe 1206 coupled to a vacuum pump (not shown).
A control unit 1207 such as a microcomputer is provided to control the entire ADRIP apparatus of FIG. 12. Particularly, the control unit 1207 receives signals from the evaporation amount sensors 1202-1S, 1202-2S and 1202-3S to control the evaporation sources 1202-1, 1202-2 and 1202-3 as well as the pressure gradient type arc discharge plasma gun 1204 and the adjusting valve 1205a. 
When the ADRIP apparatus of FIG. 12 carries out the ADRIP main-process at step 1105 of FIG. 11, the control unit 1207 operates the pressure gradient type arc plasma gun 1204 to receive Ar gas and He gas and generate arc discharge plasma 1208 at a high electron density and at a low electron temperature. Also, the control unit 1207 operates the adjusting valve 1205a to introduce O2 gas into the vacuum chamber 1201. As a result, a large amount of active atoms and active molecules such as oxygen radicals are generated. On the other hand, Pb vapor, Zr vapor and Ti vapor generated from the Pb evaporation source 1202-1, the Zr evaporation source 1202-2 and the Ti evaporation source 1202-3 react with the above-mentioned active atoms and active molecules and are deposited on the wafer 1203a heated at about 500° C. As a result, PbZrxTi1-xO3 with a composition ratio x is formed on the wafer 1203a. 
The piezoelectric actuator of FIG. 10 is heated by the ADRIP pre-process and the ADRIP main-process at steps 1104 and 1105 to about 500° C. As a result, as illustrated in FIG. 13, Ti of the Ti adhesive layer 3 is diffused into the Pt lower electrode layer 4. Also, Pb of the PZT piezoelectric layer 5 reacts with the Pt lower electrode layer 4 and is further diffused into the Ti adhesive layer 3 and the silicon oxide layer 2.
As illustrated in FIG. 14, which shows the element concentration distribution within the piezoelectric actuator excluding the PZT components of FIG. 10 after the formation of the PZT piezoelectric layer 5, the boundary between the silicon oxide layer 2 and the Ti adhesive layer 3 and the boundary between the Ti adhesive layer 3 and the Pt lower electrode layer 4 are obscure, so that Ti with strong affinity for oxygen gas, nitrogen gas and hydrogen gas absorbs these gases which react with Ti to harden the Ti.
In order to suppress the obscurity of the above-mentioned boundaries, a manufacturing method as illustrated in FIG. 15 may be adopted instead of the manufacturing method as illustrated in FIG. 11. In FIG. 15, note that step 1501 is provided instead of step 1104 of FIG. 11. That is, at step 1501, an ADRIP pre-process is carried out in an oxygen atmosphere. However, even by step 1501 of FIG. 15, as illustrated in FIG. 16, which shows the element concentration distribution within the piezoelectric, actuator excluding the PZT components of FIG. 10 after the formation of the PZT piezoelectric layer 5, the boundary between the silicon oxide layer 2 and the Ti adhesive layer 3 and the boundary between the Ti adhesive layer 3 and the Pt lower electrode layer 4 are still obscure. Note that FIG. 17 shows the element concentration distribution within the piezoelectric actuator including the PZT components of FIG. 10 corresponding to FIG. 16 where the depth corresponds to the etching time of FIG. 17.
Thus, in the prior art piezoelectric actuator of FIG. 10, since atoms other than Ti atoms cannot be sufficiently oxidized in order to secure a clear boundary between the Ti adhesive layer 3 and the Pt lower electrode layer 4, Ti of the Ti adhesive layer 3 and Pb of the PZT piezoelectric layer 5 are diffused into the Pt lower electrode layer 4, and react with Pt of the Pt lower electrode layer 4. As a result, as illustrated in FIG. 18A, the crystallizability of the Pt lower electrode layer 4 greatly fluctuates so that the surface roughness would be increased. Also, as illustrated in FIG. 18B, the crystallizability of the Pt lower electrode, layer 4 within one wafer greatly fluctuates. In FIG. 18B, note that P181 indicates a portion where the crystallizability of the Pt lower electrode layer 4 is good, while P182 indicates a portion where the crystallizability of the Pt lower electrode layer 4 is bad. Therefore, as illustrated in FIG. 19A, the columnar crystallizability of the PZT piezoelectric layer 5 greatly fluctuates. Also, as illustrated in FIG. 19B, the piezoelectric constant (−d31) of the PZT piezoelectric layer 5 within one wafer greatly fluctuates. In FIG. 19B, note that P191 indicates a portion where the piezoelectric constant (−d31) is high, while P102 indicates a portion where the piezoelectric constant (−d31) is low. Further, as illustrated in FIG. 20, the surface roughness of the Pt upper electrode layer 6 greatly fluctuates. Therefore, when a DC voltage, is applied between the Pt lower electrode layer 4 and the Pt upper electrode layer 6, a strong electric field would be locally focused so that the breakdown voltage characteristics would deteriorate. Thus, the manufacturing yield would be decreased.
Further, an adhesive layer (buffer layer) of ZrO2 between an insulating substrate and a lower electrode layer is known (see: US2004/0173823A1, JP2001-088291A and JP2003-179278A). In this case, the adhesive layer on the side of the insulating substrate is Zr, while the adhesive layer on the side of the lower electrode layer is ZrO3. Therefore, since the adhesion between the insulating substrate and the adhesive layer and the adhesion between the adhesive layer and the lower electrode layer are carried out by metal-to-insulator bonding such as molecular bonding or electrostatic bonding, these adhesions are very weak. Therefore, when an ADRIP process is applied to such an adhesive layer, the interface between the adhesive layer and the insulating substrate (or the lower electrode layer) would be peeled off due to the high and low temperatures produced by the ADRIP process.